Telefunken short wave receiver type a
Photo originating from a 1941 product catalogue.
Please consider also my paper on:
“The significance of German electronic engineering in the 1930s”
Status: 31 March 2014
On 15 March 2014, at the annual rally in Rosmalen (NL), we obtained a scrap module belonging to the beautiful Telefunken receiver type KWE-a. Although, it apparently was not intact, I, nevertheless, decided to purchase it as being a nice example of Telefunken pre-war engineering. Normally, the IF (ZF) tuning section is invisible kept behind hardly accessible shielding plates. Dismantling the module is a quite difficult undertaken, facing also the danger of breaking the many genuine seals.
As to understand better what this section is about, we first take a brief look at the schematic section of the IF stage; albeit, constituting only a small part of this module.
Those familiar to IF circuitry layout may understand that this design is very comprehensive; let us call it - exceptional
One might wonder why only the upper four bandwidth ranges can be selected. The reason is - that the IF section is tuned at 250.9 kHz (centre frequency) with its (physical) limitations. Why? Please consider the next table:
at setting 1 the overall bandwidth is 8 kHz and it does not implement audio filtering
at setting 2 the overall bandwidth is 5.1 kHz no (audio) filtering, with BFO is LSB*
at setting 3 the overall bandwidth is 3.4 kHz no (audio) filtering, with BFO is LSB
at setting 4 the overall bandwidth is 1.7 kHz no (audio) filtering, with BFO is LSB
at setting 5 IF bandwidth kept at 1.7 kHz and audio filter bandwidth 0.9 kHz, LSB
at setting 6 IF bandwidth kept at 1.7 kHz and audio filter bandwidth 0.4 kHz, LSB
at setting 7 IF bandwidth kept at 1.7 kHz and audio filter bandwidth 0.2 kHz, LSB
at setting 8 IF bandwidth kept at 1.7 kHz and audio-bandwidth 0.2 kHz BFO = USB
* LSB means lower sideband, USB upper sideband
Viewing the KWEa front panel and functions reproduced from the GAF manual D.(Luft) T. 4403
The overall dimensions: width 69.2 cm - height - 27.4 cm - deep 34.6 cm, weight 42 kg!
Viewing the KWEa from above
The module we are currently dealing with is the one on the left-hand side.
Viewing the KWEa RX from the rear
(Our module is now on the right-hand side)
A nice example of its sound concept - which receiver of those days (year of its commercial availability 1938) has compensated for the BFO off-set when calibrating the RX scale?
The valve most left is the third IF amplifier. Right of it the separate AVC- or call it AGC (AVR) automatic gain control stage. In those days, there have been not many receivers around equipped with a separate gain-controlling stage (amplifier). The third valve, noticed from the left-hand side, is the detector stage (Audion). Directly followed by the selectable audio filtering; finally proceed by the headphone amplifier.
Viewing the the IF filter section (cover plates being removed)
The quite big coils are containing rather heavy adjustable dust-cores. Quite curious, the Al coil shielding is mounted at the cover plate.
The centre coils being removed, as, for whatever reason, the previous owner has cut off all their connecting wires. The long brass strips are part of the selector/switching system; based upon a Siemens&Halske patent DE318011, year of application 1918
The lower R and C banks are those used for loading the IF (ZF) filter coils, as to control the RX IF bandwidth performance. Please notice, that when the coupling between the two coil sections being varied this consequently will also causing a de-tuning of the filter performance. And, bear also in mind, that the L/C ratio is also changing - hence the Q-factors with all their implications (notice the schematic circuit shown up this page).
Viewing the module a bit more from the top
Viewing the opened IF- and BFO stage from the rear. Please, consider the Siemens & Halske selector-switch patent DE318011 again
The (pluggable) dual BFO quartz 250 and 251,8 kHz on the right-hand side is lacking. The various colours used at the ceramic capacitor surface is indicating their Tc factor as well as the particular ceramic property where it is made from. Please notice, for example, the Hescho guide of 1939
Viewing the detector and audio stage. On the far left-hand side we notice the BFO section
This picture definitely shows that the receiver construction is very neat.
Viewing the AVC or AGC amplifier stage. Up on the far right-hand side we see a settable selector, allowing, at will, a short- or longer lasting automatic gain response. In my perception also a quite unique feature
Not good visible, but the grey block capacitor marked '3 x' is definitely manufactured after, say, mid 1943 and is of a far lower quality standard than the rest of the blocking Cs; like the one mounted vertically left of the brown filament choke. Even today, 99.9 % of the hermetically sealed-off types full-fill their genuine specifications! However, the application of this particular capacitor might not causing too much troubles when it fails, as the gain control is likely not being operated in most cases.
The transformer like coil is a choke used in combination with filaments, each valve is having its individual one. The KWEa is fed from 2 V dc and needs a 90 V anode battery (or using the mains power supply NA6 or that like), they therefore use 11 battery valves all of type RV2P800; constituting also a typical German feature, using wherever possible a single (military) valve type. Viewing the images backwards you will recognise these devices at various places. Its purpose is to keep the + filament line separated from their common 2 V supply, as the filament is acting as a cathode.
Finally viewing the module more from the front
The potentiometer is having two sections, one controlling loudness and the other one is for controlling the HF receiver gain. Why? Simply, because when the gain control being switched-on the audio channel is having full amplification and the audio gain is being controlled by adjusting the HF gain, but using instead the knob what otherwise is adjusting loudness. The HF gain-control (311) is at the right-hand side of the receiver; being, however, out-off function when automatic gain control is operated. The handle up on the far left-hand side is selecting whether AVC- or without gain control. The heavy switch lever is meant for BFO on or off (Tg / Tn, which means: telegraphy or telephony).
It has to be noticed though, this receiver concept was mainly meant for telegraphy (CW) reception, which was, in those days, the general means of wireless communication.
By Arthur O. Bauer